Fluid-filled variable focus lenses are well known. They normally consist of a liquid-filled chamber, at least one face of which is formed by a transparent flexible membrane. As liquid is introduced into or removed from the chamber, the flexible membrane is deformed, and its curvature accordingly changes. This change in curvature leads to a change in the optical characteristics and power of the lens. The power of the lens can thus be varied simply by varying the amount of liquid in the chamber.
One type of prior art variable focus lens is disclosed, for example, in WO 96/38744. In this document, the amount of fluid in the lens is adjusted by inserting a syringe through a bung, and using the syringe to add or withdraw fluid. If this lens is to be used in spectacles, the spectacle frames for the lenses must allow the lens to be accessed by the syringe, and it will be appreciated that use of this lens can be rather inconvenient in some circumstances.
For variable focus lenses for use in some types of spectacles, for example reading glasses, the required range of correction is quite small, and correction can be achieved across the entire range using a relatively small amount of liquid. This small amount of liquid can be stored in a relatively small reservoir.
According to one proposal, the lens can be formed from a ring to which a flexible membrane is attached to form one wall of a liquid filled cavity, and the reservoir can be provided in the interior of a hollow extension integral to the ring. As the reservoir is then integral to the ring, there is no need to attach or detach syringes or the like to allow adjustment of the lens. Further, there is no need to provide a separate duct connecting the reservoir with the lens cavity; in the past, such separate ducts have been prone to damage, and have been capable of interfering with the folding of the spectacles.
The volume of the reservoir can be varied, for example by forming the reservoir as a cylinder and moving a piston in the cylinder. It has been proposed to provide control means in the form of a handwheel operatively connected to the piston, such that rotational movement of the handwheel causes translational movement of the piston and thus causes the optical characteristics of the lens to vary.